In order to properly demodulate an amplitude modulated (AM) data communications signal, a local oscillator is required in the receiver that operates in phase and frequency synchronism with the carrier signal. If a carrier signal is transmitted, a local oscillator can be phase-locked onto the carrier detected at the receiver. However, carrier transmission is an inefficient use of transmitted power since the carrier contains no useable data information. If a suppressed carrier (SC) AM transmission is used, a different phase-locking technique is necessary.
In a double sideband suppressed carrier (DSB-SC) system, a local oscillator in the receiver can be phaselocked to the carrier signal using a so-called Costas loop. (See Costas, J. P. "Synchronous Communications", Proc. IRE vol. 44 December 1956). However, the Costas loop is not suitable for phase recovery in single sideband suppressed carrier (SSB-SC) systems or vestigial sideband suppressed carrier (VSB-SC) systems having very small vestige. In general, this is due to the dependence of the quadrature components on the in-phase components of the baseband signal. Reference is made to "Application of a Costas Loop to Carrier Recovery for VSB Communication Systems," Ebert and Ho, ICC73, Conference Record, Vol. II, June 1973, for a further discussion of this point.
In U.S. Pat. No. 3,675,131 (Pickholtz) there is described a technique for carrier phase recovery in an SSB-SC system. In this system, an in-phase estimated carrier signal is generated by a local oscillator having an error signal input which is used to vary the phase of the estimated carrier. The error signal in question is proportional to [f.sup.2 (t) + f.sup.2 (t)] Sin .phi., where f(t) and f(t) are the inverse Fourier and Hilbert transforms of the baseband spectrum, respectively, and .phi. is the phase error between the received carrier and the estimated carrier. The technique taught in the Pickholtz patent relies on the assumption that the Hilbert transform of the estimated baseband waveform is the same as the waveform obtained by demodulating the received waveform using an estimated carrier in phase quadrature with the in-phase estimated carrier. This assumption is only valid if the transmission channel is nearly perfect.